Add support for IAT obfuscation (disabled by default).

When enabled, inter-packet delay will be randomized between 0 and 10
ms in 100 usec intervals.  As experiences from ScrambleSuit (and back
of the envelope math based on how networks work) show, this is
extremely expensive and artificially limits the throughput of the link.

When enabled, bulk transfer throughput will be limited to an average of
278 KiB/s.

1 files changed, 45 insertions, 9 deletions

diff --git a/obfs4.go b/obfs4.go
index f32c222..3bbccb3 100644
--- a/obfs4.go
+++ b/obfs4.go
@@ -50,6 +50,8 @@ const (
 
 	maxCloseDelayBytes = maxHandshakeLength
 	maxCloseDelay      = 60
+
+	maxIatDelay        = 100
 )
 
 type connState int
@@ -67,6 +69,7 @@ type Obfs4Conn struct {
 	conn net.Conn
 
 	lenProbDist *wDist
+	iatProbDist *wDist
 
 	encoder *framing.Encoder
 	decoder *framing.Decoder
@@ -444,13 +447,38 @@ func (c *Obfs4Conn) Write(b []byte) (n int, err error) {
 		return 0, err
 	}
 
-	// Send the frame(s).
-	_, err = c.conn.Write(frameBuf.Bytes())
-	if err != nil {
-		// Partial writes are fatal because the frame encoder state is advanced
-		// at this point.  It's possible to keep frameBuf around, but fuck it.
-		// Someone that wants write timeouts can change this.
-		return 0, err
+	// Spit frame(s) onto the network.
+	//
+	// Partial writes are fatal because the frame encoder state is advanced
+	// at this point.  It's possible to keep frameBuf around, but fuck it.
+	// Someone that wants write timeouts can change this.
+	if c.iatProbDist != nil {
+		var iatFrame [framing.MaximumSegmentLength]byte
+		for frameBuf.Len() > 0 {
+			iatWrLen := 0
+			iatWrLen, err = frameBuf.Read(iatFrame[:])
+			if err != nil {
+				return 0, err
+			} else if iatWrLen == 0 {
+				panic(fmt.Sprintf("BUG: Write(), iat length was 0"))
+			}
+
+			// Calculate the delay.  The delay resolution is 100 usec, leading
+			// to a maximum delay of 10 msec.
+			iatDelta := time.Duration(c.iatProbDist.sample() * 100)
+
+			// Write then sleep.
+			_, err = c.conn.Write(iatFrame[:iatWrLen])
+			if err != nil {
+				return 0, err
+			}
+		    time.Sleep(iatDelta * time.Microsecond)
+		}
+	} else {
+		_, err = c.conn.Write(frameBuf.Bytes())
+		if err != nil {
+			return 0, err
+		}
 	}
 
 	return
@@ -508,7 +536,7 @@ func (c *Obfs4Conn) SetWriteDeadline(t time.Time) error {
 // DialObfs4 connects to the remote address on the network, and handshakes with
 // the peer's obfs4 Node ID and Identity Public Key.  nodeID and publicKey are
 // expected as strings containing the Base64 encoded values.
-func DialObfs4(network, address, nodeID, publicKey string) (*Obfs4Conn, error) {
+func DialObfs4(network, address, nodeID, publicKey string, iatObfuscation bool) (*Obfs4Conn, error) {
 	// Decode the node_id/public_key.
 	pub, err := ntor.PublicKeyFromBase64(publicKey)
 	if err != nil {
@@ -528,6 +556,9 @@ func DialObfs4(network, address, nodeID, publicKey string) (*Obfs4Conn, error) {
 	// Connect to the peer.
 	c := new(Obfs4Conn)
 	c.lenProbDist = newWDist(seed, 0, framing.MaximumSegmentLength)
+	if iatObfuscation {
+		c.iatProbDist = newWDist(seed, 0, maxIatDelay)
+	}
 	c.conn, err = net.Dial(network, address)
 	if err != nil {
 		return nil, err
@@ -554,6 +585,7 @@ type Obfs4Listener struct {
 	nodeID  *ntor.NodeID
 
 	seed *DrbgSeed
+	iatObfuscation bool
 
 	closeDelayBytes int
 	closeDelay      int
@@ -587,6 +619,9 @@ func (l *Obfs4Listener) AcceptObfs4() (*Obfs4Conn, error) {
 	cObfs.isServer = true
 	cObfs.listener = l
 	cObfs.lenProbDist = newWDist(l.seed, 0, framing.MaximumSegmentLength)
+	if l.iatObfuscation {
+		cObfs.iatProbDist = newWDist(l.seed, 0, maxIatDelay)
+	}
 	if err != nil {
 		c.Close()
 		return nil, err
@@ -629,7 +664,7 @@ func (l *Obfs4Listener) NodeID() string {
 // ListenObfs4 annnounces on the network and address, and returns and
 // Obfs4Listener. nodeId, privateKey and seed are expected as strings
 // containing the Base64 encoded values.
-func ListenObfs4(network, laddr, nodeID, privateKey, seed string) (*Obfs4Listener, error) {
+func ListenObfs4(network, laddr, nodeID, privateKey, seed string, iatObfuscation bool) (*Obfs4Listener, error) {
 	var err error
 
 	// Decode node_id/private_key.
@@ -655,6 +690,7 @@ func ListenObfs4(network, laddr, nodeID, privateKey, seed string) (*Obfs4Listene
 	rng := rand.New(newHashDrbg(l.seed))
 	l.closeDelayBytes = rng.Intn(maxCloseDelayBytes)
 	l.closeDelay = rng.Intn(maxCloseDelay)
+	l.iatObfuscation = iatObfuscation
 
 	// Start up the listener.
 	l.listener, err = net.Listen(network, laddr)